Color penetration cathode ray tube systems have been used for displays where high resolution is desired, an example being given in U.S. Pat. No. 4,099,088 issued Jan. 4, 1978, to H. M. Hart in which the shifting between colors is achieved by varying the cathode to anode voltage at the phosphor screen. The switching time from color to color can be reduced by reducing the portion of the anode whose voltage is changed while retaining large portions of the post deflection anode structure at substantially constant voltage.
When such structures are used in large cathode ray tubes having substantially flat face plates, correction must be made for the focus if sharp spot size and registry of spots or different electron velocity beams is to be maintained. Otherwise, misregistry of beams of different velocities produced by different anode voltages causes current impurity or misregistry of colors in an objectionable manner in peripheral regions of the cathode ray tube screen.